A security fastener including a body having a circumferential periphery and an end wall, a curvilinear groove in the end wall, a plurality of circumferentially spaced depressions extending inwardly into said circumferential periphery from said end wall and having a bottom wall and first and second side walls, junctions having radii between the bottom wall and the side walls. One of the side walls extends at a greater angle to the axis of the fastener than the other side wall, which is engaged by the driving tool. A driving tool for driving both the foregoing security fastener and a conventional hexagonal fastener including a body having a bore with a plurality of axially extending splines for selectively engaging either the above-mentioned side wall of the depression of the security fastener or a plurality of sides of the hexagonal fastener.